Heavy or residual petroleum oils of high sulfur content such as petroleum asphalt are less valuable because of their serious impact on the environment when directly burnt as fuel. Therefore, these heavy or residual petroleum oils are generally used as useful industrial feed-stocks after transformed into lighter products by cracking. As one of these technologies, the following process scheme of thermal cracking and its facility are mentioned.
As a processing scheme for thermal cracking of heavy petroleum oil, there is a proposed scheme consisted of a cracking furnace and a reaction vessel in which the heavy oil fed through the cracking furnace is introduced into the reaction vessel to be thermally cracked (refer to Patent Documents 1 to 4). The heavy petroleum oil fed to the reaction vessel is directly contacted with superheated steam of 500 to 700° C. that is blown into from the bottom of the reaction vessel. Heavy petroleum oil in the reaction vessel is thermally cracked and transformed into aliphatic hydrocarbons rich gaseous cracked substances and poly-aromatics rich petroleum pitch. The gaseous cracked substances are discharged from the top outlet of the reaction vessel together with steam and introduced into the distillation tower for separation.
As represented typically in the Patent Documents 1 to 4, there is proposed a process scheme in which one cracking furnace and two reaction vessels are provided. Heavy petroleum oil fed through the cracking furnace is introduced into the first reaction vessel, and then introduced into the second reaction vessel after feeding to the first reaction vessel is completed. Feeding to the reaction vessels is switched alternately so that the heavy petroleum oil is thermally cracked semi-continuously.
In these thermal cracking processes, the heavy petroleum oil fed to the reaction vessel is directly contacted with superheated steam blown into from the bottom of the reaction vessel and thermally cracked to produce aliphatic hydrocarbons rich gaseous cracked substances and poly-aromatics rich petroleum pitch. The gaseous cracked substances are discharged from the top outlet of the reaction vessel together with steam and introduced into the distillation tower for separation. However, since the thermal cracking process in reaction vessels is batch-wise operation, the amount of gaseous cracked substances discharged from the reaction vessels is not constant but fluctuated through cycles. Accordingly, the flow-rate of the gaseous substances charged to the distillation tower is varied greatly with variation range of not less than 25% occasionally. This fluctuation of the flow-in quantity of the gaseous substances causes unstable operation of the distillation tower and results in inferior separation performance or reduced operation of the upstream cracking section.
In addition, gaseous substances flown into the distillation tower contains carried-over pitch that is the precursor of coke. In order to prevent the precursor of coke from contaminating into the product oil it is necessary to supply adequate quantity of wash oil in the lower section of the tower matching the flow-in quantity of the gaseous substances. In this situation, when there is a fluctuation of the flow-in quantity of the gaseous substances, it is unpractical to change the quantity of wash oil depending on the feed fluctuation. It is also economically undesirable to supply constant quantity of wash oil matching the maximum flow-in quantity of the gaseous substances.
Further, since the latent heat of vaporization and sensible heat of gaseous substances is recovered simultaneously in the distillation tower, quantity of steam generated from the heat recovery system is fluctuated. Consequently, operation of the refinery boiler plant is affected by this fluctuation. Therefore, uniformity of the flow-in quantity of the gaseous substances to the distillation tower has been desired.
Further, in order to increase the cracking capacity of heavy oil, it can be usual way to increase the capacity of without increasing number of equipment such as furnace, reaction vessel and distillation tower. However, range of fluctuation of the feed to the distillation tower might be enlarged more in the following revamping cases.                Capacity increase with the same number of reaction vessel        Addition of new cracking train of 2 reaction vessels with simultaneous feed to the first reaction vessel.        
The term “cycle” used in the present invention means the interval from starting of the feed through the furnace to the first reaction vessel to the completion of feed to the second reaction vessel and re-starting of feed to the first reaction vessel for each cracking train.    [Patent Document 1] JP07-116450 B (1995)    [Patent Document 2] JP54-15444 B (1979)    [Patent Document 3] JP57-15795 B (1982)    [Patent Document 4] JP63-38076 B (1988)